Method and means for comminuting solid particles



G. A. FINK Feb. 17, I953 METHOD AND MEANS FOR COMMINUTING SOLID PARTICLES Filed July 16, 1948 1:5 mus c r012 Blob/ER zz INVENTOR. 650861 A F NK BY d.

K/ZITTORNE) Patented Feb. 17, 1953 FJU'NIZTED STATES PATENT OFFICE L METHOD AND ZME'ANS FOR COMMINUTING SOLID PARTICLES George A. Fink, Flushing, N. Y.

.Applica'tionJuly 16, 1948, SerialNo. 39,106

; QCIaims.

""I'he;.'present invention relates to a method of :and "means for comminuting particles of solid .materials.

It is.'an"'obje'ct of theinvention "to provide a jfmethodof reducing'theparticle size of-solidmateams in" a" novel and expeditious manner.

"lIt .is'anotheriobje'otof the invention to provide "aimethodbf "and means for" obtaining" particles "Z desired order of magnitude without resortingkto'iiepeatd classifying;"(e. gfscreeningl proc- "sses.

Whefisolid materials'arecomminuted in conventional devices such 'ias ball mills, it will generally be found upon screening that a certain pervcentageof. the particles have not been reduced to the.desird'sizewwhile others' 'are considerably ..=smaller.-ithanf the jnorm. While the particles of \Ltheifirstrgroupmay be reprocessed and thereby Tultimately Lbrought 1 to 11 the proper dimensions, .Lthose bf the. second group cannotv be enlarged and thus may beirretrievably lost for some-pur- Mposes.

It.is,-. accordingly, a further object of my inven- ..'tion tolprovide a method of and means for comminuting particles by subjecting them to forces *WhiOhw-Will be efiective to reduce the size :of the :iparticle only .where the latter exceeds a predetert-Lmined order of magnitude; but will be-substan- :ireti'ally inefiective for, smaller particle sizes.

5 Still. another. object -is-to; provider a: method of ii-Bind means for, comminuting particles without inourring.;.the.. riskof contamination; such "as will generally occur .wheraeasziin a ball mill, thejparticles omehinto 'violent contact: with the surfaces :ser extraneousmatter.

According to the present invention, 1 subject the particles to be comminutd toa forcewhich will tend 'to-rotate "them at increasing speeds about-their own-centers of gravity,- such rotation giving rise tocentri-fugal-stresseswithin the particle whiolr-will be proportional to the square of the'ra'diusofgyration and, henceg-roughly to the cross-sectional area of the' particle. The particles *arepr'eferably subjected to this force for a suitable length of time calculated-so that'all particles r 'exce'dinga givenorder or; magnitude will attain 'n'tlie critical spedat which they are disintegrated. *order -to' understand the operation of "an arrangem'entaccording to my invention,- itwill be *helpful'to-consider eaclr particle-as the short-circuited armature of an alternating-currentmotor,

-"t'ating'electron!agn'eticfield. It oan? be shown that the torquetapp-lied t0 suChi armaturewiu retma'inithe "sam'efevenfif thev laxisofi theiarrnature the value: 2 i being therre'quency ofi thero'tating' 'fi'e'ldjthis f'otationigiving:rise tofcenti'ifugal forces proportional to the squ'ar er the" effective diameter of the armaturewhiehxou a precalcu- Iable rninimum 'radius will oilercome the c'ohesive forces of the material and b'reali the armature in two.

v Applying these principles to the particles un'der consideration, "and assumingthe' shape of each particle te -be" that of a sphere' of radius 1 it can be shown that an alternating electromagnetic field offrequency'f will induce currentsin these particles resiflting' in apower loss the" dielectric constantand the power factori respectively, er the material of which the "particles are composed. The'res'ulting' torque M "-niay be If the spherical particlehas a density then itsmomeht bfine'rtia will be'g'ivn by the formula celeration can bedeterinind as lByrass'uminga rconfstant acceleration "we obtain necessary to bring a particle up to the angular Neloeity. wz2n-f.

, Actually, neithentheltorque norlthe' centrifugal .stresses-set. up within the particle are susceptible l to accuratescalculation, oW-ingi to the irregular .shape .ofthe-particle the centrifugal force, 'however, will .be .of the order rofmagnitude -of the .stress existing inathin ring or hoop whose diameterriscequallto theldiameter. ofia circle of "an area equal to. thecmaximum cross section "of the 1 particle .irL thetplane of "rotation "diameter we may again..assumeas 12p. "Thiscefit'rifu'gal force isgiven byztheirormulare mzwwzm which 1 We may. substitute {for {2 asthemmimum radius at, .whichldisintegration. s'houldoccur. JI he critito S, thelatterlbeingfthe. tensile strength: of the material.

cal. frequency .can then-.be'foiind by equating F 1111f the. accompanyin 5' drawin Ilhave I'shown Several arrangements for carrying the invention into practice. Fig.1 is an"'e1e'vation "and Fig'. 2

a schematic top view of a first arrangement according to the invention while Fig. 3 shows the invention in a modified form.

Referring first to Figs. 1 and 2, there are shown four electrodes H, l2, l3 and Hi defining a chute I within which particles i may fall freely while being subiected to the influence of a rotating electric field. In order to produce the rotating field, electrodes H and i3 are connected directly across a high-frequency generator it which latter also energizes electrodes I2 and [4 over a 90 phase shifter l1. Electrodes H and M are grounded as indicated at l8, electrode H having been removed in Fig. 1 in order to show the interior of chute l0.

The particles [5 may enter the chute by way of a suitable screen l9 and may be collected at the bottom by a vessel 20. It will be appreciated that the chute I0 should be wide enough to insure a substantial uniform electric field across the path of the falling particles 15, the length of the chute bein selected so that even the largest particles will find time to be sufiiciently accelerated, as previously discussed, before reaching the bottom of the chute.

From the foregoing it will have been understood that the frequency of generator l6 determines the size of the particles ultimately obtained, higher frequencies resulting in smaler particles.- High frequencies. on the other hand, tend to discriminate against large particles since the latter may not find time to accelerate to the final angular velocity 2w). To overcome this difficulty, I have illustrated in Fig. 3 an arrangement whereby particles may be exposed to a rotating field for an extended length of time without requiring a chute of impractical proportions. Fig. 3 also illustrates how the particles may be treated in a rotating magnetic rather than electric field. Chute l0a is here formed by four pole shoes of which only three, indicated at 42a, l3a and Ma, respectively, are shown, each pole shoe being an extension of a core 19' of an electromagnetic coil 20 connected to a suitable source of alternating current (not shown). Particles l5 are introduced int the chute from below, by means of an injector 2|, being carried upwards by a stream of air produced by a blower 22. The particles are collected in a tube 23, of inverted U-shape, to the end of which a bag 24 is attached. The porosity of the bag should be such that air from the blower 22 may readily pass therethrough while the particles will be retained, this principle being generally applied in vacuum cleaners.

The arrangement of Fig. 3 has the additional advantage of counteracting the aforesaid discrimination against large-size particles, inasmuch as the smaller, lighter particles will pass more quickly through the chute |0a whereas the heavier ones will float for a longer period. If necessary, the air stream may be replaced by some inert gas in order to prevent oxidation of the particles during treatment, and similarly the process of Figs. 1 and 2 may be carried out in an inert atmosphere or in vacuum.

It will be understood that the formulae given above apply to the production of an electromagnetic field by electrostatic means, as illustrated in Figs. 1 and 2, but that similar formulae may be developed for the magnetic arrangement shown in Fig. 3. In the latter case, the permeability of the material of the particles will be a factor in lieu of the dielectric constant and the power factor thereof.

If the particles are composed, for example, of calcium carbonate (marble) which is a dielectric material sometimes used for insulating purposes, the frequency necessary for breaking up particles of a given diameter, say 10 microns, may be determined from the above formula for the centrifugal stress by inserting for S the value of 2000 p. s. i. or 1.38-10 dynes/cmF. This gives us a frequency of 2.26 megacycles per second which is easily obtainable. By inserting the values of 2.72 for 'y, .04 for a, 13,000 for E and 8.3 for e in the above formula for the time of acceleration, the latter will be established at .078 second, a very convenient value which corresponds to a free drop of roughly 3 cm. If the potential of 13,000 volts per cm. is too high, or if materials having a lower dielectric constant and/or power factor are used, the required length of the chute may be reduced by artificially increasing the power factor of the particles as by suitable coating, moistening or heating the same.

It will have become apparent from the foregoing that the loss factor of the particles to be disintegrated, i. e. the product of their dielectric constant e and their power factor a, must be determined to enable proper selection of field strength, frequency and other parameters previously indicated. This loss factor can be established experimentally, on the basis of the above formula for the torque M, whence or, approximately, 5-10 -M/E V wherein V is the volume of a spherical particle of radius p.

Inasmuch as this loss factor is thus independ ent of frequency, it can be determined with the aid of a rotating field of any convenient frequency, using the same principles as those on which the invention is based. Practical tests, which incidentally proved the correctness of the underlying theory, have been carried out on different materials, yielding the following results:

Material: Loss factor Aluminum oxide (coarse) 0.12 Aluminum oxide (fine) 0.26 Magnesium oxide 1.44 Ferric oxide (rouge) 1.20 Silicon carbide, mesh 0.13 Slate (chips) 0.19 Soapstone (block) 0.11 Talc, powder (average particle size about 15 microns) 0. 72

These tests were carried out as follows:

The materials to be tested, usually in powder form, were placed in a cup made of a material known to have an extremely low power factor, in the actual case polystyrene. The cup had a depth of 4.9 cm. and an inside diameter of 2.25 cm. The cup was suspended by a calibrated torsion wire approximately in the center of a plate assem bly as shown in Fig. 2, to which alternating fields of about 2500 vol-ts, shifted in phase by as previously described, were applied. With a plate spacing of 5.25 cm. the field strength E, whose frequency was the commercial one of 60 cycles per second, was of the order of 500 volts/cm.

As was to be expected in view of the low power factor of polystyrene, no measurable torque was exerted by the field upon the empty cup, hence no corrections had to be applied to the torque measured when the material to be tested was particulafirfatrial at fiai bleparticles ranging diameter from taro nierbfis are to be comminuted ma field as above set forth; 1;; eg'qf 1%;000vclts-per cm. and v e nd, until substantially odd to a allest par ticles to be broken up, i. e. those having the limiting diameter of microns, attain a speed suffi-cientfor their disintegration. With respect to the largest particles, i. e. those of 40 microns in diameter, it will be noted from the foregoing formula for the centrifugal force that they need only be accelerated to an angular velocity of about w/4 to attain the desired result. From the acceleration formula previously given it will further be apparent that this angular velocity will be attained by said large particles after a time of about 4T, or approximately 0.3 second; this represents a free drop of roughly 45 cm. The resulting fragments, having a mean diameter of microns, must be further accelerated to an angular velocity of about w/2 which, again from the formula mentioned, may be computed as occurring within a time T or 0.78 second approximately. This step yields particles cf about 10 microns in diameter which, having already an angular velocity of w/2, only need to be accelerated over a time T/2 to attain the velocity w, the total procedure thus requiring a time of a little above 0.4 second corresponding to a drop of about 90 cm. It will thus be apparent that a chute length of less than 1 m. is required to disintegrate particles of the size mentioned to one-eighth of their original linear extent yet that at the same time all particles of lesser size will be broken up to not less than the same general order of magnitude. The foregoing example applies, apart from the fact that it is based on a number of approximations, only in a vacuum or at very reduced pressure, since otherwise a decrease in acceleration due to the effects of 55 friction will have to be taken into account.

In passing it may be noted that whereas the disintegrating forces will have their maximum effect in a plane perpendicular to the field and will have no effect in a direction parallel thereto, the action of the centrifugal forces will tend to position each original particle or resulting fragment with its major dimension in such perpendicular plane, whereby disintegration will be substantially uniform in all dimensions.

It is understood that the specific values herein referred to have been given in an illustrative and not in a limiting sense, and that the invention may be embodied in arrangements other than those specifically described and illustrated without departing from its spirit or exceeding its scope as defined in the objects and in the appended claims.

I claim: 7

1. The method of comminuting solid particles 76 0 said particles; rota ivelyentrain d bysa attain an angular velocity suflicient to caiis dis integration of substantially all particles exceed ing a given order of magnitude.

2. The method of cdrnminuting solid particles which oomprisesthe step-s of producing a rotating electromagnetic fielder high frequency. said'fi'eld having a substantially vertical axis of rotation; dropping the particles to' becomminuted through said field, andsdco-ntrolling the tim'of transit of said particles through said field= that said par ticles,--rotatively entrained by said field; attain an" angular velocity's-ufficientto caiise disintegfa tier-i of substantially all particlesexcee'dinea given order of magnitude.

3. The method according to claim 1, wherein the particles to be comminuted consist of dielectric material, comprising the further step of increasing the power factor of said particles prior to droppin them through said field.

4. The method of comminuting solid particles Which comprises the steps of producing a rotating electromagnetic field at high frequency, said field having a substantially .vertical axis of rotation, floating the particles to be comminuted through said field in an upward direction, and so controlling the time of transit of said particles through said field that said particles, rotatively entrained by said field, attain an angular velocity su-fficient to cause disintegration of substantially all particles exceeding a given order of magnitude.

5. An arrangement for comminuting solid particles, comprising a source of high frequency alternating current, at least four electrode plates connected across said source with a progressive phase shift, thereby setting up a rotating electrostatic field of an angular velocity exceeding that at which particles of a given material exceeding a predetermined order of magnitude will be centrifugally disintegrated, said electrode plates forming a chute coaxial with the axis of rotation of said field, and feed means adapted to pass the particles to be comminuted through said chute in spaced relation to the walls thereof and at a rate allowing said particles to approach the angular velocity of said field.

6. The arrangement according to claim 5 wherein said chute is substantially vertically disposed, said feed means comprising a screen located at the top of said chute.

'7. The arrangement according to claim 5, wherein said chute is substantially vertically disposed, said feed means comprising blower means adapted to produce an upward-traveling gas stream within said chute and injector means adapted to introduce said particles into said gas stream adjacent the bottom of said chute.

8. The method of comminuting solid particles of a given material which comprises the steps of producing an electromagnetic field rotating about a substantially vertical axis at an angular velocity exceeding that at which those of said particles which exceed a predetermined order of magnitude will be centrifugally disintegrated, introducing the particles to be comminuted into said field, creating an upwardly directed gas stream in said field, and floating said particles on said gas stream until said particles, rotatively entrained by said field, attain an angular velocity approaching that of said field.

9. The method of comminuting solid particles of a given material of density '7 and tensile strength 6 which comprises the steps of producing an electrostatic field rotating at an angular velocity in excess of a value w given by the formula wherein r is the diameter of the smallest particles to be comminuted, introducing the particles 'to be comminuted into said field, thereby causing said particles to be rotatively entrained by said field, retaining said particles in said field for a sufiicient length of time to allow them to reach an angular velocity between an and the velocity of said field, and removing the particles from the field after at least a major portion of those exceeding said predetermined order of magnitude have been disintegrated.

' GEORGE A. FINK.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,043,349 Ostwald Nov. 5, 1912 1,417,189 McCarthy May 23, 1922 1,791,100 Lykken Feb. 3, 193 1,894,106 Lehrack Jan. 10, 1933 2,079,155 Cri'tes May 4, 1937 OTHER REFERENCES The Electrically Driven Magnetically Supported Vacuum Type Ultracentrifuge by C. Sk'arstrom and J. W. Beams, vol. 11, pp. 398-403, Review of Scientific Instruments. 

